Dead Forests Release Less Carbon Into Atmosphere Than Expected

Vast swaths of forest are succumbing to pine beetle outbreaks. (Photo: David Moore/UA School of Natural Resources and the Environment)

Massive tree die-offs release less carbon into the atmosphere than previously thought, new research led by the University of Arizona suggests.

Across the world, trees are dying in increasing numbers, most likely in the wake of a climate changing toward drier and warmer conditions, scientists suspect. In western North America, outbreaks of mountain pine beetles (Dendroctonus ponderosae) have killed billions of trees from Mexico to Alaska over the last decade.

Given that large forested areas play crucial roles in taking carbon dioxide out of the atmosphere through photosynthesis and turning it into biomass, an important question is what happens to that stored carbon when large numbers of trees die.

"A question we are looking to answer is, 'How does the carbon dioxide released from the forest into the atmosphere change as you have large scale tree mortality over time?''' said second lead author Nicole Trahan, a postdoctoral researcher at the University of Colorado, Boulder.

According to co-author Russell Monson, who is the Louise Foucar Marshall Professor in the UA School of Natural Resources and the Environment, forests affect the carbon budget of the atmosphere through two dominant processes: photosynthesis, by which plants take carbon dioxide out of the atmosphere and lock it up in organic compounds, and respiration, by which plants and soil microbes release carbon dioxide back into the atmosphere. The balance of these processes determines whether a particular forest is a carbon source or a carbon sink.

After a massive tree die-off, conventional wisdom has it that a forest would go from carbon sink to carbon source: Since the soil microbes are still around, they are expected to release large amounts of the greenhouse gas carbon dioxide into the atmosphere, where it is thought to accelerate climate change.